Luminescent material



Patented Nov. 19, 1940 UNITED i STATES LUMINESCENT MATERIAL SampsonIsenberg, Chicago, Ill., a'ssignor to General Luminescent Corporation,Chicago, Ill., a

' v corporation of Illinois No Drawing.

a Serial N 6 Claims.

My invention relates to ground luminescent materials and methods ofproducing the same, and'more particularly to luminescent materials ofthe Willemite type.

Large numbers of materials which exhibit luminescence when in thepresence of exciting rays, such as relatively short electro-magneticwaves, have been known for many years. Comparatively recently, interestin luminescent ma- 10 terials, and the synthetic manufacture and usesthereof, has attracted attention due tothe commercial application ofsuch materials to surfaces exposed to exciting waves, such as thoseproduced by mercury discharge tubes.

Despite the relatively large amount of development work looking towardimprovement in luminescent materials, andaverments made with respect tosuperior results obtained by certain treatments, in general,investigators have not given suflicient consideration to all'of theconditions which may surround the preparation and use of thesematerials. The source and character of the exciting radiation is ofconsiderablev importance and a material which, is eminently 5satisfactory under one set of conditions may not be satisfactory underconditions where the exciting ray is not the same in either itscharacter or intensity.

My invention is particularly concerned with luminescent materialsadapted to be coated onto the inside surface of space, discharge tubes,primarily mercury discharge tubes, such as the mercury-argon,mercury-krypton or similar tubes used for advertising or lightingpurposes.

Luminescent materials may be produced: so that when merely crushed; butnot ground, they will pass a 200 mesh screen. While unground material ofthis general mesh size may be used, and has been extensively used, intube coating operations, much better results may, in general, beobtained with-a finer mesh material. While a finely. ground material maybesuitable for many purposes, it is not satisfactory for excitaApplication October 7, 1940,

production of improved ground luminescent materials of the Willemitetype. c

Other objects of my inventionwill be clear from the followingspecification:

I have found that, when producing a lumi- 5 nescent material of thesynthetic Willemite type, that is, an'orthosilicate type of materialemploying manganese as an activator, the proportion of activatoremployed is important not only with respect to the exciting source towhich the ma- 10 terial is to be exposed, but also with respect to thetreatment which it is to undergo. More specifically, in the productionof a ground manganese-activated orthosilicate, I have-found that theproportion'of activator should be greater 15 than that which willproduce maximum bril liancy in the same matrix material, if unground andexposed to the same exciting source.

In general, in carrying out my invention, I may, for example, employstoichiometric proporg0 tions of materials which, on reaction, willproduce the orthosilicate. Together. with these materials, Lemploymanganese as an activator, in proper proportions as hereinafterdescribed. The manganese may be introduced as a salt of 25 manganousacid, or in other suitable form.

The materials generally referred to in the preceding paragraph areplaced together and ground in a. ball mill for from about two to abouttwentyfour hours, depending upon the particle size of 30 the introducedmaterials. In general, but only by way of illustration, I prefer to runa five-gallon ball mill about fifty revolutions per minute. Thisprocedure produces highly satisfactory results. By way of example, ifthe starting ma- 3'5 terials introduced into the ball mill areapproximately 300 mesh and the ball mill is operated at about 50revolutions per'minute, approximately two hours grinding will suffice,

One of the important features of the grinding o is to obtain an intimatemixture of the constituents. I find that when I employ a compound ofmanganese which is colored, for example, a perman'ganate, as one of thesources of manganese,

considerable assistance-is afforded byvisual in- 45 spection of thecolor distribution in checking the character of mixture obtained due tothe ball milling. A desirable source of manganese is a mixture ofmanganous chloride and zinc permanganate, a mixture comprisingapproximately 5% 50 i of the latter and of the former producing adequatecolor for my purpose.

After'the materials in propor' proportion to produce the activatedluminescent material have been ball milled, the resulting mixtureis-placed in a crucible which is not affected'by the reacting 1materials, such as, for example, a fire clay or i platinum crucible.

above 1200 degrees C; The temperature, the pro- ;portion of materialheated, and its distribution in the furnace are factors to be taken intocon-I siderationin determining thetime of heating.

In order that those skilled in the art may uncharacter of the excitingradiation by means of derstand more fully the features of my presentinvention and know how to practice the same, I j disclose below someillustrative examples therechloride These materials, as selected, arepreferably in the Example 1.6 pounds of zinc oxide, 3 pounds of silicicacid and 12.7 ounces of manganous (MnClzAHzO) are mixed together.

j form of dine powders, firi'e' enough, for example, 1 to pass a 300mesh screen. The mixture is in- 'tro'ducedinto a ball mill, and the'ball mill is 1 operated until thorough mixing of the finely dividedstarting materials has been accomplished. 1 Two to three hours isusually an adequate amount 1 of time. The mixture'of' materials soobtained is then introducedintofire-clay crucibles" and I. fired toreaction. With these" particular starting materials, a temperature of"1220 degrees C. 1 is suitable and the time for firing is about three v Ito'four hours or slightly longer. ""A sintere'd re- 1 actionproduct isobtained which is readily broken ground product; since it is readilybrokenup, in many instances even with the fingers. This materialmay-then be ground, particularly in a 1 .ball mill, to produce a veryfinely divided material suitable for coating tubes by various meth-,

, comprises an excellent coating material for mer-. 1 cury-argon tubes.

ods. The material, ground c0400 meshor finer,

The product so, produced contains I about 5 2 manganese asan activator.This prodnot, as initially produced, that is, prior to being ground orball mille'd,"in general, produces a considerably less brilliant lightwhen'coated onto 1 the inside surface of a mercury-argon tube than doesa material comprising the-identical matrix material but containing onlyabout 1%% I i product,- on the. other hand, does not suffer apjmanganese. .Ongrinding, however, the material produced in accordancewith the example sufl fers only very slight loss of brilliancy, whereasi the identical matrix material, with less activator, suffers a markedloss of brilliancy on prolonged 1 ounces ofberyllium oxide, all in theform of fine powders of about 300 mesh, are ball milled for about threehours, .and the resulting mixture heated.to reaction in'fire claycrucibles for four hoursat 1150 degrees C. on cooling, the mate rialproduced is. crushed and passed through a- 3 200-mesh screen. The 200mesh material is then i balimilled to 400 mesh or jfiner,,andabrilliant' commerciallyvery desirable material is produced.Thesamematrix material, activated with less The crucible is then intro-3 'duced into a furnace where uniform temperature and freedom fromcontamination are substan-.

\ milling.

i The conditions of measurement stance the same.

manganese, however, while initially (in the un- 1 ground state) ofgreater brilliancy than the material of the example, will suffer amarked loss of brilliancy when subjected to prolonged grinding.

Thus it will be seen that I haveproduced an improved, finely groundluminescent material of the Willemite or orthosilicate type, at leastfine enough to pass a ADO-mesh screen, and having very much greaterbrilliancy than a ground material comprising the same matrix butcontainf, ing less manganese. :tion preferably contains at least about1.75% manganese,- and may contain as much as about The material of myinven- 5% manganese, depending upon the source and which it will beexcited. ,By employing suitable starting materials-and firing at atemperature wand for suflicient time to bring about a reaction,specifically above approximately 1000 degrees C. 1 but not appreciablyabove 1200 or 1250 degrees C., ,I produce amaterial which is readilycrushed to 200-mesh without grinding, and then ground to at leastWithout preciable loss of j, brilliancy.

- The manganese employed is calculated ,onthe basis of the metal in allof the percentages given.

The'exa'ct form. of the manganese in the final product is notfullyknown,-but there-isprobably a mixture of various-forms, When I'refer inthe claims to the amountsof manganese present,

therefore, I' contemplate the'total amount thereweight of theluminescent material as awhole.

The luminescent material maybe ball milled with other substances, suchas binders, to prepare'a mixture for application 'toa tube. The

relationship of the percentage "of manganese to grinding is afactor sofar as all modifications of Willemite t'ypes'of materials are concerned.The

* preferred or optimum range-of manganese should be increased by betweenabout 25% and 75% 'deprefer to employ as a standard procedure from about1.75% to about 2.5% of manganese as an activator, and preferably betweenabout 2% and When a 1.25% m'anganeseproduct is produced,

it willhave a measurable brilliancy somewhat greaterth'ana-2.5%'manganese product. 'When the 1.25% product is ball milledextensively, howi ever, it suffers a marked loss of brilliancy, and itslight-asemitted shifts slightly toward the violet endof the spectrum.The 2.5% manganese preciable loss of brilliancy after extensive ball Asanexample, to illustrate the brightness relationship underthe variousconditions discussed, the case of a. green Willemite maybe considered.By actual test, I have found that a; zinc orthosilicate activated withabout 1.5% of' tions to 55. On the other hand, the samematrixmaterialactivated with 2.5% manganese may initially, in theungroundstate, have, a bright- .ness value of 90, but, after being groundgto 400mesh, will still show. a brightness ,value of ,82.

, The results obtainable by'the practice of were in each ininventionhold true whether the,basis of calculation be with reference topercentage by weight or calculations of molar ratios. When the molecularweight of the matrix material is heavy, the actual weight percentage ofmanganese may be decreased. To produce a ground Willemite type ofmaterial in accordance with my invention for excitation by amercury-argon mixture, I find that good results are obtained when themolar proportion of manganese is .102 moles per mole of matrix material,whereas, in general, .06 moles of manganese or somewhat less producesthe greatest brightness in the unground state.

I have described my invention in considerable detail so that thoseskilled in the art may understand and practice the same. The inventionis limited, however, only by the scope of the appended claims.

This application is a continuation-in-part of my prior application,Serial No. 260,893, filed March 10. 1939.

What I claim as new and desire to protect by, Letters Patent of theUnited States is:

l. A finely divided luminescent material of the Willemite type ground tofine size after the production thereof,- said material comprising anorthosilicate activated with at least,1.75% of manganese, calculated asthe metal, and being at least of sufficient fineness to pass a 400 meshscreen, said material having its brilliancy, when applied to a mercurydischarge type, substantially unimpaired by the grinding operationrequired to produce a fine state of subdivision.

2. A finely divided luminescent material of the Willemite type ground tofine size after the production thereof, said material comprising anorthosilicate activated with from about 1.75% to about 5% of manganese,calculated as the metal, and being at least of sufficient fineness topass a 400 mesh screen, said material having its brilliancy, whenapplied to a mercury discharge tube, substantially unimpaired by thegrinding operation required to produce a fine state of subdivision.

3. A finely divided luminescent material of the Willemite type ground toa fine size after the production thereof, said material comprising anorthosilicate activated with from about 2% to about 2 of manganese,calculated as the metal, and being at least of sufiicient fineness topass a 400 mesh screen, said material having its brilliancy, whenapplied to a mercury discharge tube, substantially unimpaired by thegrinding operation required to produce a fine state of subdivision.

4. The method of producing a luminescent material of the Willemite typefor activation by an exciting radiation in a luminous discharge tubewhich comprising mixing together, in finely divided form, proportions ofmaterials which on firing will produce said Willemite type of material,and a proportion of manganese 25% to greater than that'proportiondetermined to be necessary to produce maximum brilliancy when the finalmaterial in an unground state is subjected to said exciting radiationunder the same conditions, heating the mixture at a temperature above1000 degrees C. but not appreciably above 1200 degrees C. and from twoto five hours to produce a luminescent material, crushing the resultingmaterial enough to pass it through a 200 mesh screen and then grindinthe material until it will pass a 400 mesh screen,

the presence of the greater amount of manganese than necessary toproduce maximum brilliancy in an unground material under the conditionsserving to prevent appreciable loss of brilliancy and change inluminescent tone quality during grinding.

5. The method of producing a luminescent material of the Willemite typefor activation by an exciting radiation of a luminous discharge tubewhich comprises mixing together proportions of reacting materials whichon firing will produce zinc orthosilicate, and a proportion of manganese25% to 75% greater than that proportion determined to be necessary toproduce maximum brilliancy when the final material in an unground stateis subjected to said exciting radiation under the sameconditions,heating said mixture to reaction but not to a temperature which willproduce a sintered mass, whereby a material of somewhat less brilliancythan the maximum producible in such amaterial is produced, and thensubjecting said material to prolonged grinding to produce a finelydivided product, all of which will pass a 400 mesh screen, whereby theresulting ground material will have much greater brilliancy than asimilarly ground material containing initially the smaller proportion ofmanganese.

6. The method of producing a'finely divided luminescent material of theWillemite type comprising a matrix material and a manganese activator,for excitation when coated on a mercury discharge tube, which methodcomprises mixing together finely divided materials which on firing willreact to produce a manganese activated orthosilicate, the proportion ofmanganese, calculated as the metal, being between about 2% and 2 /2% byWeight of the matrix material, firing said materials above 1000 degreesC. for a sumcient length of time to produce a completely reactedmaterial, crushing said material to pass a 200 mesh screen, and thengrinding the crushed material until it will pass a LOO-mesh screen, theproportion of manganese employed having the effect of preventing theusual lossof brilliancy in the grinding step.

SAMPSON ISENBERG.

